Polarity-reversal catalysis of hydrogen-atom abstraction reactions: concepts and applications in organic chemistry

The rates and selectivities of the hydrogen-atom abstraction reactions of electrically-neutral free radicals are known to depend on polar effects which operate in the transition state. Thus, an electrophilic species such as an alkoxyl radical abstracts hydrogen much more readily from an electron-rich C-H bond than from an electron-deficient one of similar strength. The basis of polarity-reversal catalysis (PRC) is to replace a single-step abstraction, that is slow because of unfavourable polar effects, with a two-step process in which the radicals and substrates are polarity-matched. This review explores the concept of PRC and describes its application in a variety of situations relevant to mechanistic and synthetic organic chemistry.